vim-patch:8.1.0027: difficult to make a plugin that feeds a line to a job

Problem:    Difficult to make a plugin that feeds a line to a job.
Solution:   Add the nitial code for the "prompt" buftype.
f273245f64

src/nvim/buffer_defs.h

@@ -791,6 +791,9 @@ struct file_buffer {
                                 // are not used!  Use the B_SPELL macro to
                                 // access b_spell without #ifdef.
 
+  char_u *b_prompt_text;        // set by prompt_setprompt()
+  Callback b_prompt_callback;   // set by prompt_setcallback()
+
   synblock_T b_s;               // Info related to syntax highlighting.  w_s
                                 // normally points to this, but some windows
                                 // may use a different synblock_T.